As generally known in the art, a Cold Cathode Fluorescent Lamp (CCFL) has various advantages including low power consumption, small heat radiation, high luminance, long lifespan, etc. because it can be operated by low current. The CCFL is now widely used as a backlight of a Liquid Crystal Display (LCD). A high Alternating Current (AC) voltage of 1 to 2 KV is necessary in order to turn on such a CCFL, and a DC-AC converter is usually used in order to provide such a high AC voltage.
FIG. 1 illustrates a circuit of a conventional DC-AC converter.
As shown, the conventional DC-AC converter includes a transformer TX1, a Direct Current (DC) power source 21′, a bias/reference voltage generator 23′ for generating bias and reference voltages necessary for internal operation from the DC power source 21′, a switching unit 28′ including four Field Effect Transistors (FETs) from switch A to switch D for providing current paths within the transformer TX1 by switching the voltage V1 according to driving signals, an LCD panel 22′ including a CCFL operated by the transformer TX1, a protection circuit unit 26′ for detecting an output voltage OVP and providing a sweeping stop signal when the detected output voltage exceeds a reference voltage, a frequency sweeper 27′ for generating a rectangular pulse of 50% duty-cycle by performing frequency sweeping until the output voltage OVP exceeds the reference voltage before the step signal is input from the protection circuit unit 26′ in an open lamp state, a feedback control unit 24′ for comparing the feedback voltage from the protection circuit unit 26′ with the reference voltage and controlling a switch-on time of the switching unit based on a result of the comparison, and a driving circuit unit 25′ for providing a driving signal to the switching unit 28′ according to the rectangular pulse of the frequency sweeper 27′ and a switch-on time control signal of the feedback control unit 24′.
The protection circuit unit 26′ includes a comparator 26A′, a timer 26B′, and an electric current sensor 26C′. The comparator 26A′ determines if the lamp is open or not by comparing the CMP signal and a voltage signal from the LCD panel 22′ with the reference signal and provides a stop signal to the frequency sweeper 27′. The timer 26B′ has a time out period set in advance and is started when the detected voltage exceeds the reference voltage. When the timer has been operated during the time out period, the comparator 26A′ provides the stop signal. The electric current sensor 260′ controls the frequency sweeper 27′.
U.S. Pat. No. 6,259,615 discloses a detailed example of such a DC-AC converter as described above.
The conventional DC-AC converter having the construction as described above employs a phase-shift scheme, i.e. a serial operation scheme, according to which a pair of switches including switch A and switch B are first sequentially operated by using the signal from the frequency sweeper, and the other pair of switches including switch C and switch D are then sequentially operated by using the feedback signal thereof.
However, such a conventional DC-AC converter requires a complicated control method and uses a 50% pulse frequency sweeper. Therefore, the conventional DC-AC converter requires complicated design and high manufacturing costs.
Further, most manufacturers currently produce DC-AC converters employing the phase-shift scheme or similar schemes, which thus have a high possibility of patent conflict occurring between them. Therefore, there is a need for a DC-AC converter, which can be controlled by a simple control method, can be designed in a simple manner, and is inexpensive to manufacture, while avoiding patent conflicts.